High blood sugar damages your body through two broad mechanisms: it pulls water out of your cells and tissues in the short term, and it chemically alters proteins throughout your body over the long term. A fasting blood sugar above 100 mg/dL is already in the prediabetes range, and at 126 mg/dL or higher, it meets the threshold for diabetes. The higher your levels climb and the longer they stay elevated, the more widespread the damage becomes.
What Happens in Your Body Right Away
When glucose builds up in your blood beyond what your cells can absorb, your kidneys try to flush the excess out through urine. Glucose pulls water along with it, which is why frequent urination and intense thirst are the earliest signs of high blood sugar. In severe cases, this water loss can be dramatic. People in a hyperglycemic crisis can lose up to 9 liters of water from their circulation, enough to cause dangerously low blood pressure and organ failure if not corrected.
This fluid shift affects every cell. As water leaves your tissues to compensate for the concentrated blood, cells become dehydrated. You may feel fatigued, lightheaded, or have trouble concentrating. Your body releases hormones to try to hold onto water in the kidneys, but if you aren’t drinking enough to keep up, dehydration spirals quickly.
How Glucose Chemically Damages Your Tissues
The longer-term destruction is more insidious. Excess glucose in the bloodstream reacts with proteins, fats, and even DNA through a process sometimes called the Maillard reaction (the same type of chemical browning that happens when you toast bread). Glucose attaches to proteins and, through a series of intermediate steps, forms permanent structures called advanced glycation end products, or AGEs.
AGEs cause problems in two ways. First, they create cross-links between structural proteins like collagen and elastin, essentially gluing them together. This stiffens blood vessel walls, thickens the membranes that line your capillaries, and makes tissues less flexible. Second, AGEs bind to receptors on cell surfaces that trigger inflammation and oxidative stress, a cascade that further damages blood vessel linings and accelerates plaque formation. The effect is cumulative: the longer blood sugar stays elevated, the more AGEs accumulate, and the stiffer and more inflamed your blood vessels become.
Damage to Small Blood Vessels
The smallest capillaries in your body are especially vulnerable. In the eyes and kidneys, high blood sugar causes the tiny support cells that wrap around capillaries to die off. These support cells (called pericytes in the eyes and podocytes in the kidneys) act like scaffolding. When they’re lost, the capillary walls become leaky and fragile.
In the retina, this leakage leads to small hemorrhages, swelling, and eventually the growth of abnormal new blood vessels that can cause vision loss. In the kidneys, the same process damages the filtering units. Capillary obstruction and the loss of those support cells allow protein to spill into the urine, a hallmark of declining kidney function. Both conditions progress silently for years before symptoms appear, which is why blood sugar control matters long before you notice any problems.
Damage to Large Blood Vessels
High blood sugar doesn’t just harm capillaries. It accelerates atherosclerosis, the buildup of fatty plaque in major arteries, through a specific chain of events. Excess glucose triggers the cells lining your arteries to produce sticky surface molecules that attract white blood cells. Those white blood cells burrow into the artery wall, gobble up oxidized cholesterol particles, and become “foam cells,” the core building blocks of arterial plaque.
At the same time, high glucose reduces your blood vessels’ ability to produce nitric oxide, the molecule that keeps arteries relaxed and flexible. Without enough nitric oxide, arteries constrict and stiffen. Combined with the AGE-driven cross-linking of collagen in vessel walls, the result is arteries that are both narrower and harder, raising the risk of heart attack and stroke. This is why cardiovascular disease is the leading cause of death among people with chronically elevated blood sugar.
Nerve Damage
Your nerve cells are particularly sensitive to high glucose because they absorb sugar without needing insulin. When glucose floods into nerve cells, it gets shunted into a backup processing route called the polyol pathway, where it’s converted into sorbitol. Sorbitol doesn’t cross cell membranes easily, so it accumulates inside the nerve cell, drawing in water and causing swelling. This same pathway also depletes the cell’s antioxidant defenses, leaving it exposed to oxidative damage.
The result is peripheral neuropathy, which most commonly starts as tingling, numbness, or burning in the feet and hands. Over time, nerve damage can spread to the digestive tract (causing nausea or unpredictable digestion), the bladder, and the cardiovascular system (making it harder for your body to regulate heart rate and blood pressure).
Weakened Immune Response
High blood sugar also impairs your immune system’s first responders. Neutrophils, the white blood cells that rush to infection sites and injured tissue, become less effective at mobilizing when glucose is elevated. Research has shown that hyperglycemia reduces the signaling molecules that call neutrophils out of the bone marrow and into circulation. With fewer neutrophils arriving at the scene, wounds heal more slowly and infections take hold more easily. Normalizing blood sugar with insulin has been shown to restore neutrophil recruitment, which underscores how directly glucose levels control immune function.
When High Blood Sugar Becomes an Emergency
Chronically elevated blood sugar causes gradual damage over months and years, but acutely high levels can become life-threatening within hours. Diabetic ketoacidosis (DKA) occurs when the body, unable to use glucose for energy, starts breaking down fat at a dangerous rate. The byproducts of that fat breakdown, called ketones, make the blood acidic. DKA is typically diagnosed when blood sugar is above 250 mg/dL, blood pH drops below 7.3, and ketones are present in the blood or urine. Symptoms include nausea, vomiting, abdominal pain, fruity-smelling breath, and confusion. It can occur in people with type 1 or type 2 diabetes, and it requires emergency treatment.
It’s also possible to develop DKA with blood sugar below 250 mg/dL, a condition called euglycemic ketoacidosis. This is less common but means you can’t rely on blood sugar readings alone to rule it out.
How Blood Sugar Levels Are Measured
If you’re trying to understand where you stand, there are a few key numbers to know. Fasting blood sugar (measured after at least 8 hours without eating) is normal below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes. At 126 mg/dL or higher on two separate tests, it’s diabetes.
A glucose tolerance test measures your blood sugar two hours after drinking a sugary solution. Normal is below 140 mg/dL, prediabetes falls between 140 and 199 mg/dL, and 200 mg/dL or higher indicates diabetes.
The A1c test gives a broader picture by measuring how much glucose has attached to your red blood cells over the past two to three months. An A1c of 5% corresponds to an average blood sugar of about 97 mg/dL. At 7%, the average is around 154 mg/dL. By the time A1c reaches 10%, average blood sugar is approximately 240 mg/dL. Every percentage point increase on the A1c scale translates to roughly 29 mg/dL of additional average glucose, which helps put daily readings in context with the long-term trend.